MAINT: upload all wheels simultaneously

[andyfaff]

Follows scipy-release behaviour in having all wheels being uploaded simultaneously in a separate step.

scipy-release seems to be working well after having transitioned to this model.

We might need to add an extra environment called none to satisfy the logic of when to upload on a manual trigger.

[andyfaff]

In this repo settings I added an extra 'none' environment that can only deploy when it's built on the main branch.
The workflow_dispatch (i.e. manual trigger) can upload to nightly if the none environment is used during the triggering. There will be a further approval before upload occurs.

The scheduled cron job should be unaffected.

[rgommers]

There will be a further approval before upload occurs.

That doesn't seem to be configured - and that's fine, it's unnecessary in my opinion. If you trigger a workflow on main, that's what you want, so no need for an extra gate I think.